It's well known that WebP provides a much better quality/size ratio along with alpha mask support, so using it for the cache-db could provide two advantages - smaller size and/or better quality. And lossy low-quality compression can still store the alpha channel, which can used when drawing the thumbnails.

I think it should be selected by default in thumbnail cache options, even without ZIP/JPG (although these should be internally supported for old cache-db files).